1. Field of the Invention
This invention relates to a method for removing toxic and otherwise dangerous impurities from gas streams and, more specifically, this invention relates to a new injection method for removing elemental mercury (Hg0) and nitrogen oxides (NOx) from off-gas or flue-gas streams by reacting them with vaporized oxidizing agents.
2. Background of the Invention
Mercury (Hg) was identified as a hazardous air pollutant in Title III of the 1990 Clean Air Act Amendments. It has assumed singular importance for the electric utility industry, particularly after the EPA concluded that coal-fired boilers generate a significant fraction of the total man-made mercury emissions in the United States. Indeed, the EPA has announced its intention to regulate Hg emissions from coal-fired power plants and is expected to propose regulations by 2003.
While utility sources are widely dispersed and seem extremely dilute by typical air-pollution standards, mercury (Hg) can have a lifetime of many months or even years in the atmosphere. Mercury is thus subject to long-range transport, which makes its control a national and international issue. Indeed, mercury, deposited in terrestrial/aquatic environments, tends to “bio-accumulate” in living organisms at concentrations considered neuro-toxic to animals (including man) at the top of the food chain.
Early estimates of utility control costs for mercury using duct injection of activated carbon range from about $25,000/lb to $70,000/lb of mercury removed. These costs can be contrasted with those for NOx control, which tend to be less than $2.50/lb of pollutant removed, and even that is usually considered expensive. (NOx is more nitric oxide (NO) with the remainder being nitrogen dioxide (NO2).) With these high costs for “add-on” controls, techniques that utilize existing flue-gas cleaning systems for mercury and NOx removal would be desirable from both an economic and operational perspective.
Dry particulate-matter collectors, such as electrostatic precipitators, have not been shown to be very effective at capturing mercury, but some wet scrubbers installed for flue-gas desulfurization (FGD) have yielded high removals. However, the performances obtained with different scrubber systems have been highly variable with values that have ranged from about 10% to over 80%. Neither technology has any appreciable effect on NOx emissions.
In general, the fate of mercury and other trace elements liberated in the combustion process is influenced by the type of boiler, the operating conditions, other species present in the flue gas, and the type of flue-gas cleanup (FGC) system. Mercury is a particular problem because it belongs to a group of elements and compounds denoted as Class III, which exist primarily in the vapor phase within the boiler and the subsequent FGC system. It can also exist in several chemical species. The valences of mercury and chlorine underscore the likelihood of the covalent association of these two elements. In particular, the presence of chlorine in coal means that mercury can be found in both the elemental and oxidized forms, with the relative amounts depending on such factors as the original ratio of chlorine to mercury in the coal, gas temperatures, and length of exposure of the gas at various temperatures.
Another confounding factor in flue gas clean up is sulfur presence. All coals have a measurable sulfur content. Oxidation of coal (through combustion) will produce sulfur oxides (SOx), which consist mostly of sulfur dioxide (SO2) with small amounts of sulfur trioxide (SO3) often present. Sulfur oxides have a detrimental effect upon such oxidizing agents as chlorine and bromine used to remove mercury. For example, while a liquid solution of chloric acid has been used as an oxidizing agent to remove elemental mercury from flue gases, its oxidizing effect is impeded by the presence of sulfur oxides found in the gases. Specifically, U.S. Pat. No. 5,900,042 awarded to the instant inventor Mendelsohn, et al. on May 4, 1999 discloses a method to convert elemental mercury in a gas stream to soluble mercury compounds. In that patent, removal of elemental Hg varied from 26.9% to 69.6% depending upon the experimental conditions. Hg removal in one set of experiments decreased to 48.2% from 69.6% upon the addition of SO2 to the simulated effluent gas.
U.S. Pat. No. 5,328,673 awarded to Kaczur, et al. on Jul. 12, 1994 discloses a process for removing NOx and SOx from gaseous streams by contacting the gaseous stream with liquid chloric acid.
A disadvantage with the prior methods is that mixing of reactants is hindered when the liquid-phase oxidant contacts the gas-phase effluent. Another disadvantage is the increased energy loss which occurs when cooler liquid-phase oxidants are introduced into a heated reaction zone.
U.S. Pat. No. 5,785,932 awarded to Helfritch on Jul. 28, 1998 discloses a catalytic reactor for oxidizing elemental mercury vapor contained in a flue gas. The method teaches that the condensed mercuric oxide may be captured by a conventional particle collector.
A need exists in the art for a completely gaseous method for simultaneously removing NOx and elemental Hg from flue streams. The percentage removal of elemental Hg and NOx must not be diminished appreciably by the presence of other flue gas components such as SOx . A combined process would have significant economic advantages over the use of two separate technologies for the removal of these two hazardous species.